Reactive sintering of ceramic lithium-ion solid electrolytes

ABSTRACT

A method of forming a solid, dense, hermetic lithium-ion electrolyte membrane comprises combing an amorphous, glassy, or low melting temperature solid reactant with a refractory oxide reactant to form a mixture, casting the mixture to form a green body, and sintering the green body to form a solid membrane. The resulting electrolyte membranes can be incorporated into lithium-ion batteries.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityto U.S. application Ser. No. 16/523,327, filed on Jul. 26, 2019, whichis divisional application of U.S. application Ser. No. 13/306,011, filedNov. 29, 2011, now U.S. Patent No. 10,411,288, and claims the benefit ofpriority thereto under 35 U.S.C. § 120, the contents of both of whichare relied upon and incorporated herein by reference in their entirety.

BACKGROUND

The present disclosure relates generally to the formation of ceramiclithium-ion solid electrolytes, and more specifically to a reactivesintering process for forming dense, hermetic electrolyte membranes.

Solid electrolytes, also known as fast ion conductors, are materialsthat can function as solid state ion conductors and can be used, forexample, in solid oxide fuel cells and lithium-ion batteries. In alithium-ion battery, for example, lithium ions move from a negativeelectrode to a positive electrode during discharge (and back whencharging) via the solid electrolyte. The solid electrolyte, such aslithium aluminum titanium phosphate (LATP), can conduct lithium ionthrough vacancies in the LATP crystal lattice. In Li-ion batteries, thesolid electrolyte membrane can provide a hermetic barrier between theanode and the cathode in order to prevent the anode and cathode fromsharing a common electrolyte solution.

Thus, important to the development of Li-ion batteries is theavailability of dense, conductive lithium-ion electrolyte membranes. Amajor challenge for such membranes is the desire to sinter suitablematerials to sufficient density such that the membrane is hermetic whileproviding sufficient conductivity and economy. Conventional hermeticmembranes, for example, which are commonly made using a glass-ceramicprocess, can be made dense and hermetic, but typically at the expense ofother attributes such as conductivity and cost. A challenge facing theconventional glass-ceramic process is the requirement that the desiredcomposition form a stable glass.

In view of the foregoing, it would be desirable to develop a process forforming dense, hermetic, Li-ion conductive ceramic electrolyte membraneswithout such a limitation to stable glass formation. As used herein, ahermetic membrane is substantially impervious to the diffusion ofliquids or gasses.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theinvention as described herein, including the detailed description whichfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention and together with the description serve to explain theprinciples and operations of the invention.

SUMMARY

Disclosed herein is a reactive sintering method for forming a Li-ionconductive ceramic electrolyte membrane. The process involves reactivesintering of at least two solid state reactants. In embodiments, thereactive components are powders that are combined, and heated tosimultaneously react the components and densify the reaction product.The heating step can be used to form the reaction product into amembrane using a process such as tape casting.

A method of forming a solid lithium-ion electrolyte membrane comprisescombining an amorphous, glassy, or low melting temperature solidreactant with a refractory oxide reactant to form a mixture, casting themixture to form a green body, and sintering the green body to form asolid membrane. As used herein, a glassy material has a softening pointof less than 850° C., a low melting temperature solid reactant has amelting temperature less than or equal to 850° C., and a refractoryoxide has a melting temperature of greater than 850° C. Thus, in variousembodiments the reactants include a glassy material and a ceramicmaterial. The refractory oxide may be an amorphous material, including aglass. In a complementary embodiment, the reactants include at least oneamorphous material, at least one glassy material, at least one lowmelting temperature material that are combined with at least onerefractory oxide. In each of the embodiments, at least one of thereactants is an amorphous, glassy or low melting temperature solidreactant and at least one of the reactants is a refractory oxide.

Each of the reactants can comprise a powder material having, forexample, a submicron particle size distribution. In an example method,the reactive sintering temperature is less than 1100° C., e.g., lessthan 1000° C.

Disclosed also are solid lithium-ion electrolyte membranes having acomposition represented by the formulaLi_(1+x-y)M_(x)M′_(2-x-y)M″_(y)(PO₄)₃, wherein M is a 3⁺ ion, M′ is a 4⁺ion, and M″ is a 5⁺ ion. In the foregoing, 0≤x≤2 and 0≤y≤2. In anexample embodiment the product composition is a lithium metal phosphatehaving the NaZr₂(PO₄)₃ (“NZP”) crystal structure. For example,embodiments relate to dense, hermetic Li_(1.4)Al_(0.4)Sn_(1.6)(PO₄)₃lithium-ion electrolyte membranes.

The resulting membrane can have an average thickness of less than 200microns, e.g., less than 150 microns, where the constituent electrolytematerial can have an average grain size of less than 10 um, e.g., lessthan 1 um. For certain compositions, self-supporting ceramic membranesas thin as 30 microns can be formed.

According to a further embodiment, a solid electrolyte membranecomprises a sintered reaction product of a first powder and a secondpowder. The first powder comprises a first inorganic,non-lithium-ion-conductive glass or ceramic, and the second powdercomprising a second inorganic, non-lithium-ion-conductive glass orceramic. Thus, at least one of the first and second inorganic glass orceramic powders contains lithium in composition, but is not a lithiumion conductor. In further embodiments, none of the reactants are lithiumion conductors. As defined herein, a lithium ion conductor has aconductivity of at least 10⁻⁶ S/cm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a is a schematic representation of one embodiment comprisingthe reaction of a lithium phosphate glass (A) with titanium dioxide toproduce a water stable membrane with LiTi₂(PO)₃ as the major phase;

FIG. 2 is an illustration of the glass-forming regions in theLi₂O—Al₂O₃—P₂O₅ composition space;

FIG. 3 is an SEM cross-sectional micrograph of an LATP membrane madeaccording to an embodiment;

FIG. 4 is a family of XRD scans for example titanium phosphate powders;and

FIG. 5 is a family of XRD scans for example aluminum titanium phosphatepowders.

DETAILED DESCRIPTION

A conventional glass-ceramic approach can be used to form certainlithium metal phosphate electrolyte compositions. However, theglass-ceramic approach is inherently limited to starting compositionsthat can form a homogeneous glass, typically at temperatures in excessof 1000° C. Further, the sintering of ceramic Li-ion electrolytematerials to a density sufficient to produce hermetic membranes can bedifficult due to the vaporization of volatile lithium and/or phosphatespecies at temperatures above about 1000° C. These limitations, inaddition to restricting control of the process conditions, restrict thecompositional space available for the optimization of importantproperties like conductivity and environmental stability.

According to a disclosed method of forming a solid lithium-ionelectrolyte membrane, an amorphous, glassy, or low melting temperaturesolid reactant is combined with a refractory oxide reactant to form amixture. The mixture is then cast to form a green body, which issintered to form a solid membrane. Reaction of the amorphous, glassy, orlow melting temperature solid with the refractory oxide to produce thefinal membrane composition and densification of the structure occursimultaneously, i.e., the acts of reacting and densifying at leastpartially overlap with one another during the heating.

In the disclosed approach, the amorphous, glassy, or low meltingtemperature reactant mobilizes the diffusion of reactant components,which promotes sintering, and is entirely or substantially entirelyreacted and converted to the target Li-ion ceramic electrolytecomposition. In this way, the advantage of a mobilizing glass or lowmelting temperature liquid phase is provided without the issue ofenvironmental instability because the amorphous, glassy, or low meltingtemperature component is substantially consumed during the formationreaction that yields the product phase.

The amorphous, glassy, or low melting temperature reactant(s) cancomprise, for example, lithium phosphate glasses or lithium aluminumphosphate glasses (e.g., 39% Li₂O+11% Al₂O₃+50% P₂O₅). Additionalexample glasses include germanium phosphates (e.g., 75% GEO₂+25% P₂O₅)and amorphous aluminum titanium phosphates. Further example reactantsinclude crystallized lithium aluminum phosphate ceramics. The refractoryoxides can include various oxide ceramics such as, for example, titaniumoxide, tin oxide and germanium oxide (i.e., MO₂ oxides and ceramics).

The amorphous, glassy, or low melting temperature reactant(s) mayfurther comprise an oxide modifier such as TiO₂, GeO₂, SiO₂, B₂O₃,Fe₂O₃, Nb₂O₅ and SnO₂. An oxide modifier, if used, is limited to at most30 mol. % of the amorphous, glassy, or low melting temperature reactant.For example, the concentration of the oxide modifier can be 1, 2, 5, 10,20, 25 or 30 mol. % of the amorphous, glassy, or low melting temperaturereactant.

In embodiments, an average particle size of the respective reactants canbe less than 0.5 micron (e.g., d50≤0.5 micron). An average particle sizeof one or all of the reactants can be less than 0.5, 0.1, 0.05 or 0.01microns.

The reactive sintering can be performed at a sintering temperature ofless than 1100° C. (e.g., less than 1100, 1050, 1000, 950, 900 or 850°C.), and result in a solid membrane having a thickness of less than 200microns (e.g., less than 200, 150, 100 or 50 microns). In embodiments,the maximum processing temperature (e.g., melting temperature of theglass) is less than 1300° C. (e.g., less than 1300, 1250, 1200, 1150,1100, 1050, 1000, 950, 900 or 850° C.). By minimizing the sintering(processing) temperature, the loss of lithium or phosphate constituentscan be minimized. In embodiments, a conductivity of the electrolytemembrane is greater than 10⁻⁴ S/cm (e.g., greater than 1×10⁻⁴, 2×10⁻⁴,5×10⁻⁴ or 1×10⁻⁵ S/cm). The electrolyte membrane can be fully dense orhave a density that is at least 95% (e.g., at least 95, 96, 97, 98, 99,99.9 or 99.99%) of its theoretical density.

The invention may be better understood through the following examples.

EXAMPLE 1 LTP via LP glass and TiO₂

One approach to promoting sintering is to add excess lithium, which canresult in the formation of low melting point lithium phosphate liquidphases. This approach may also result, however, in residual lithiumphosphate phase that leaches in aqueous environments from the finalmembrane, resulting in mechanical weakness and/or membrane failure.According to embodiments, a water-stable membrane can be formed using alow melting point reactant that forms a liquid phase during sintering,but where the low melting point reactant is consumed during sintering.

Referring to the Li₂O—TiO₂—P₂O₅ composition diagram shown in FIG. 1, thewater stability of the various phases indicated in this figure has beenevaluated. Phases indicated with a filled circle are stable whereasphases indicated by an unfilled circle are unstable in water. Stabilitywas determined by exposing each phase to distilled water and measuringthe conductivity of the leachate. If a substantial increase in leachateconductivity was observed, the phase is considered unstable in water.

According to the present example, and still referring to FIG. 1, alithium phosphate glass of composition “A” (25% Li₂O: 75% P₂O₅) is madeby melting and quenching the appropriate composition. The glass ofcomposition “A” is located within a glass-forming region. The glass ismilled and added to milled TiO₂ to yield a composition nominallyequivalent to LiTi₂(PO₄)₃ with slight TiO₂ excess. The milled powdersare incorporated into a tape casting slip with suitable binders andrheological modifiers, cast to a green tape, dried, cut, released andsintered to make a ceramic electrolyte membrane. This process forms awater stable phase after the lithium phosphate is consumed in making theLiTi₂(PO₄)₃. Prior to completion of the reaction, the lithium phosphateglass provides for enhanced mobility of the reactant components,facilitating sintering and densification.

EXAMPLE 2 LATP via LAP Glass and TiO₂

A lithium-ion ceramic electrolyte with the compositionLi_(1+x)Al_(x)Ti₂, (PO₄)₃ is formed. This material, which is aluminumdoped, has been shown to have a high lithium ion-conductivity exceeding10⁻⁴S/cm.

Compared with the lithium phosphate glasses, Li₂O—Al₂O₃—P₂O₅ glasses areless hygroscopic and can be milled to submicron particle size. FIG. 2shows the glass-forming region within the Li₂O—Al₂O₃—P₂O₅ “LAP”composition space. The region “1” corresponds to a compositional rangethat forms a clear glass upon melting, while the compositions withinregions “2” form a partially crystallized glass.

In this example, an LAP glass of composition (mol. %) 29.2% Li₂O, 8.3%Al₂O₃ and 62.5% P₂O₅ (composition “B”) was made by melting a mixture oflithium carbonate, aluminum metaphosphate and phosphoric acid in theappropriate quantities. The components were mixed in a platinumcrucible, dried overnight at 250° C., and melted at 1000° C. In onecase, the liquid melt was poured onto a steel plate to quench, yieldinga partially-crystallized glass. Partial crystallization was identifiedby powder x-ray diffraction, which confirmed the formation ofcrystalline Al(PO₃)₃) in the otherwise glassy matrix. In a separatecase, in addition to pouring the liquid melt onto the steel plate, thepoured glass was roller quenched, which increased the quench rate andresulted in a clear glass.

The formed glass was broken up and dry milled to form a free-flowingpowder. The powdered glass was mixed in a solvent mixture containingethanol, butanol, propylene glycol and Emphos dispersant. The powder waswet-milled in a high-energy attrition mill to a particle size withd50<0.5 micron. Separately, nano-particle TiO₂ (Aldrich, 15 nm primaryparticle size) was prepared by mixing nano-TiO₂ in a similar solventsystem and attrition milling to an agglomerate size d50<0.5 micron.

Without wishing to be bound by theory, by providing reactants having asmall particle size, the particle size within the final membrane can becontrolled (i.e., minimized).

Solids from the two mill batches (LAP glass, nano-TiO₂) were mixed toresultant composition of Li_(1.4)Al_(0.4)Ti_(1.6)(PO₄)₃ and furthermilled to promote good mixing of the two reactant materials.Polyvinylbutyral binder and a plastisizer were dissolved into the millbatch to make a tape casting slip. In embodiments, a tape casting slipcan optionally include one or both of a binder and a plastisizer. Theslip was tape cast, dried, released and fired at 850° C. for two hoursto make a sample electrolyte membrane.

Samples from the sample membrane were cut into 1″ diameter disks andcharacterized by SEM analysis for microstructure, XRD for phasecomposition and impedance spectroscopy for conductivity. A cross-sectionSEM image is shown in FIG. 3. The membrane 300 is free-standing (i.e.,unsupported by a substrate), dense, hermetic and has an average grainsize of less than 1 micron and a thickness of about 30 microns.Impedance spectroscopy shows a conductivity of 3×10⁻⁴ S/cm. XRD showsreflections consistent with nearly pure Li_(1.4)Al_(0.4)Ti_(1.6)(PO₄)₃.

Compared with the glass-ceramic method, which can involve processingtemperatures greater than 1300° C., the relatively low processingtemperatures associated with the present example minimize lithium andphosphorus loss, which results in a more reproducible, lower costprocess.

EXAMPLE 3—LAGP

The LAP glass described in example 2 was mixed with a mill batchcontaining GeO₂ milled to a d50 of less than 0.5 microns. A tape castgreen tape was prepared in a manner similar to that used in Example 2.The tape was dried, cut, released and sintered at 900° C. for 2 hours tomake a membrane sample. Samples were cut into 1″ diameter disks andcharacterized by SEM, XRD and impedance spectroscopy. The resultingmembrane is dense, hermetic and has an average grain size of less than 1micron. XRD shows near pure Li_(1.4)Al_(0.4)Ge_(1.6)(PO₄)₃. Aconductivity of 1.8×10⁻⁴ S/cm was measured by impedance spectroscopy.

EXAMPLE 4 LASnP

The LAP glass described in Example 2 was mixed with a mill batchcontaining SnO₂ milled to a d50 of less than 0.5 microns. A tape castgreen tape was prepared in a manner similar to that used in Example 2.The tape was dried, cut, released and sintered at 900° C. for 2 hours tomake a membrane sample. Samples were cut into 1″ diameter disks andcharacterized by SEM, XRD and impedance spectroscopy. The resultingmembrane is dense, hermetic and has an average grain size of less than 1micron. XRD shows near pure Li_(1.4)Al_(0.4)Sn_(1.6)(PO₄)₃. Aconductivity of 2.1×10⁻⁴ S/cm was measured by impedance spectroscopy.

Due to the limited solubility of tin oxide in glass phases, Applicantsbelieve that the LASnP composition of the present example cannot beproduced by a glass-ceramic method.

EXAMPLE 5 Modified LAP Glass

To reduce the hygroscopicity of the starting LAP glass, and to reducemilling time by increasing the glass transition temperature of theglass, the starting LAP glass may be modified by including a fraction ofa transition metal desired in the final NASICON material. The tablebelow compares hygroscopic behavior and milling times required toachieve a d50 of less than 0.5 microns for a variety of “LAP+M” melts.In Table 1, the designation “a” in the ID column represents the amountof Al substitution, i.e., Li i-paAl_(a)M2,(PO₄)₃. The Li₂O, Al₂O₃,P₂O_(5,) TiO₂ and GeO₂ compositions are reported in mol. %. The meltingtemperature T_(m) is given in degrees Celsius. Column 9 of the Tableindicates whether the glass was hygroscopic or not. In column 10, themill time is given in hours. As seen with reference to samples 5-8, theaddition of TiO₂ and/or GeO₂ can reduce the hygroscopic behavior andreduce milling time.

TABLE 1 Modified LAP glass compositions ID Li₂O Al₂O₃ P₂O5 TiO₂ GeO₂T_(m) Hyg. mill 1 LATP 0.175 0.050 0.375 0.400 0.000 — 2 LP, a = 0 0.2500.000 0.750 0.000 0.000 1100 Yes 3 LP + G, 0.225 0.000 0.675 0.000 0.1001200 Yes a = 0 4 LAP, 0.273 0.045 0.682 0.000 0.000 1100 Yes a = 0.2 5LAP + T, 0.245 0.041 0.614 0.100 0.000 1200 No a = 0.2 6 LAP, 0.2920.083 0.625 0.000 0.000 1000 No 240 a = 0.4 7 LAP + T, 0.263 0.075 0.5630.100 0.000 1300 No 96 a = 0.4 8 LAP + G, 0.263 0.075 0.563 0.000 0.1001200 No 96 a = 0.4

In Table 1, the primary phases identified by XRD for samples 5-7 wereTiP₂O₇, Al(PO₃)₃, and TiP₂O₇, respectively. The XRD scan from sample 8was amorphous.

EXAMPLE 6 Amorphous Flame Spray Powder

In a further embodiment, hermetic LATP membranes can be made viareactive sintering from an amorphous titanium phosphate (TP) and/oraluminum titanium phosphate (ATP) material. A flame spray pyrolysis(FSP) method was used to generate amorphous TP and ATP nanopowders. Inthe FSP process, Ti, Al and P precursors, such as Ti-isopropoxide, Altri-sec butoxide, AlCl₃, trimethylphosphate, etc. are dissolved in anorganic solvent such as ethanol, isopropanol, 2-methoxyehtanol, etc. Dueto the high volatility of certain precursors, the precursorconcentration in solution can be at least 30% (e.g., at least 40%) inorder to minimize loss of the precursor material during combustion. Inan example process, the solution is sprayed through a nozzle, which issurrounded by CH₄/O₂ flame, to combust the sprayed droplets. In theflame, the precursor materials react to form the nanopowder product.

TABLE 2 Precursor and powder compositions for TP and ATP materialsSolution Powder LATP Target Sample P/Ti P/Ti P/Ti TP1 2.4 1.24 1.857 TP22.2 1.48 TP3 1.8 1.48 P/(Al + Ti) P/(Al + Ti) P/(Al + Ti) ATP1 1.95 1.031.5 ATP2 1.08 1.02 ATP3 1.27 1.44

Table 2 lists example precursor (solution) compositions together withthe corresponding powder compositions for example TP and ATP materials.XRD diffraction scans for the resulting powders are shown in FIG. 4 (TP)and FIG. 5 (ATP).

In an example synthesis, a substantially amorphous ATP powder wasfabricated by the flame spray pyrolysis technique. The ATP powdercomposition (in mol. %) was 0.1065 Al₂O₃0.518 TiO₂-0.376 P₂O₅. Theamorphous ATP nanopowder was wet mill mixed with crystalline powders ofTiP₂O₇, Li₃PO₄ and Li₂CO₃ and the mixture was attrition wet milled to aparticle size of less than 0.5 micron. The powder mixture was tape castand heated at 850° C. for 2 hr to form a hermetic LATP membrane with asubmicron average grain size. The composition of the LATP membrane (inmol. %) was 0.175 Li₂O-0.05 Al₂O₃-0.4 TiO₂-0.375 P₂O₅.

Disclosed herein is a method for making dense, hermetic, Li-ionconductive ceramic electrolyte membranes. Since a homogeneous glassstarting phase is not required, the method may be applied to a muchbroader compositional space than is accessible using conventionalglass-ceramic processes. In addition, because the reactant materialsused typically have lower processing temperatures compared to those usedin the glass-ceramics route, a lower cost process exhibiting betterprocess control is achievable, especially when volatile lithiumphosphate species are present.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “reactant” includes examples having two or moresuch “reactants” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component of thepresent invention being “configured” or “adapted to” function in aparticular way. In this respect, such a component is “configured” or“adapted to” embody a particular property, or function in a particularmanner, where such recitations are structural recitations as opposed torecitations of intended use. More specifically, the references herein tothe manner in which a component is “configured” or “adapted to” denotesan existing physical condition of the component and, as such, is to betaken as a definite recitation of the structural characteristics of thecomponent.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A method of forming a solid lithium-ionelectrolyte membrane, comprising: forming a powder mixture comprising arefractory oxide reactant and at least one of an amorphous, glassy solidreactant or a low melting temperature solid reactant, the amorphous,glassy solid reactant comprising a softening point less than 850° C.,and the refractory, and the low melting temperature solid reactantcomprising a melting temperature less than 850° C.; tape-casting thepowder mixture to form a green body; and reacting the amorphous, glassysolid reactant or the low melting temperature solid reactant to form thesolid lithium-ion electrolyte membrane, wherein: the green bodycomprises at least one of a binder and a plasticizer, and an averageparticle size of the amorphous, glassy solid reactant or the low meltingtemperature solid reactant is less than 0.5 μm, and an average particlesize of the refractory oxide reactant is less than 0.5 μm.
 2. The methodof claim 1, wherein the amorphous, glassy solid reactant or the lowmelting temperature solid reactant further comprises an oxide modifier.3. The method of claim 1, wherein the binder comprises polyvinylbutyral.4. The method of claim 1, wherein the reacting comprises sintering thegreen body at a temperature of less than 1100° C.
 5. The method of claim4, wherein the reacting comprises sintering the green body at atemperature of less than 850° C.
 6. The method of claim 1, wherein amaximum processing temperature of the method is less than 1300° C. 7.The method of claim 6, wherein the maximum processing temperature isless than 850° C.
 8. The method of claim 1, wherein the average particlesize of the amorphous, glassy solid reactant or the low meltingtemperature solid reactant is less than 0.1 μm, and the average particlesize of the refractory oxide reactant is less than 0.1 μM.
 9. The methodof claim 1, wherein the solid lithium-ion electrolyte membrane comprisesan average thickness of less than 200 μm and is free-standing.
 10. Themethod of claim 9, wherein the average thickness is at least 30 μm. 11.The method of claim 1, wherein the solid lithium-ion electrolytemembrane comprises an average grain size of less than 10 μm.
 12. Themethod of claim 11, wherein the average grain size of the constituentelectrolyte material is less than 1 μm.
 13. The method of claim 1,wherein a conductivity of the solid lithium-ion electrolyte membrane isgreater than 10⁻⁴ S/cm.
 14. The method of claim 1, wherein a density ofthe solid lithium-ion electrolyte membrane is at least 95% of itstheoretical density.
 15. The method of claim 1, wherein the solidlithium-ion electrolyte membrane is hermetic such that the membrane isconfigured to provide a barrier between an anode and a cathode of abattery.
 16. The method of claim 1, wherein the solid lithium-ionelectrolyte membrane comprises a composition represented by the formulaLi_(i+x−y)MxM′_(2−x−y)M″_(y)(PO₄)₃, wherein M is a 3⁺ ion, M′ is a 4⁺ion, and M″ is a 5⁺ ion, 0≤x≤2, and 0≤y≤2.
 17. The method of claim 16,wherein M is Al or Fe, M′ is selected from the group consisting of Ti,Sn, Nb and Ge, and M″ is Nb.
 18. The method of claim 1, wherein thesolid lithium-ion electrolyte membrane comprisesLi_(1.4)Al_(0.4)Sn_(1.6)(PO₄)₃.
 19. The method of claim 1, wherein thesolid lithium-ion electrolyte membrane comprises a NaZr₂(PO₄)₃ crystalstructure.